PHP8.1新特性之Readonly Properties（只读属性）

只读属性必须要有类型限制；

只读属性只能在构造函数中初始化赋值；

只读属性不能有默认值

构造函数中赋值

class Person
{
	public function __construct(
        // 变量的类型必须进行设置
        // 否则会报错
		public readonly string $name,
		public readonly int $age,
	)
	{	
	}
}

$person = new Person('lucy', 18);
echo $person->name, $person->age;

声明再赋值

class Person
{
	public readonly string $name;
	public readonly int $age;

	public function __construct(string $name, int $age)
	{
		$this->name = $name;
		$this->age  = $age;
	}
}

$person = new Person('lucy', 18);
echo $person->name, $person->age;

对只读属性设置默认值会报错

class Person
{
	public readonly string $name = 'lucy';
	public readonly int $age;

	public function __construct(string $name, int $age)
	{
		$this->name = $name;
		$this->age  = $age;
	}
}

$person = new Person('lucy', 18);
echo $person->name, $person->age;

报错信息：Fatal error: Readonly property Person::$name cannot have default value in